Delinquent period for charging down commercial and agricultural credits.

No definition available.

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Namespace Prefix:	fmao_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Delinquent period for charging down consumer loans

+ References

No definition available.

+ Details

Name:	fmao_DelinquentPeriodForChargingDownConsumerLoans
Namespace Prefix:	fmao_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Impaired loans classified as troubled debt restructured.

+ References

No definition available.

+ Details

Name:	fmao_ImpairedLoansClassifiedAsTroubledDebtRestructured
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Leverage ratio.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmao_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum time for reevaluation.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmao_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Mortgage loan in process of foreclosure amount.

+ References

No definition available.

+ Details

Name:	fmao_MortgageLoanInProcessOfForeclosureAmount
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Re evaluation period for real estate.

+ References

No definition available.

+ Details

Name:	fmao_ReEvaluationPeriodForRealEstate
Namespace Prefix:	fmao_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Repayment period of loan with reasonable reduction of principal balance.

+ References

No definition available.

+ Details

Name:	fmao_RepaymentPeriodOfLoanWithReasonableReductionOfPrincipalBalance
Namespace Prefix:	fmao_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Residential mortgage loan.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Unsecured consumer loans credit card credits and overdraft.

+ References

No definition available.

+ Details

Name:	fmao_UnsecuredConsumerLoansCreditCardCreditsAndOverdraft
Namespace Prefix:	fmao_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount classified as loans attributable to disposal group held for sale or disposed of. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph a

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(a))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 35

-Paragraph 49

-URI http://asc.fasb.org/extlink&oid=35742846&loc=d3e3084-111518

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 948

-SubTopic 310

-Section 30

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6490209&loc=d3e48214-111002

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4437-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

+ Details

Name:	us-gaap_RealEstateAcquiredThroughForeclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of restructured debt contingently payable based on the occurrence of future events or circumstances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 60

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6402221&loc=d3e15765-112638

+ Details

Name:	us-gaap_TroubledDebtRestructuringDebtorSubsequentPeriodsContingentPaymentsAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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- Details

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Loans - Additional Information (Detail) (USD $)	3 Months Ended
Loans - Additional Information (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgage loan	$ 20,100,000fmao_ResidentialMortgageLoan	$ 20,800,000fmao_ResidentialMortgageLoan
Impaired loans classified as troubled debt restructured	1,300,000	797,200	974,000
Troubled debt restructuring	0		0
Maximum time for re-evaluation (in months)	12 months
Re-evaluation period for real estate	2 years
Unsecured consumer loans, credit card credits and overdraft lines of credit reach	90 days
Delinquent period for charging down consumer loans	120 days
Delinquent period for charging down commercial and agricultural credits	120 days
Foreclosed residential real estate property	1,098,000	1,094,000
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Repayment period of loan with a reasonable reduction of principal balance	2 years
Other Real Estate Owned - Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Foreclosed residential real estate property	452,000
Residential real estate properties foreclosure proceedings	465,000
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	$ 457,000	$ 459,000
Good [Member] \| Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Leverage position	150.00%
3 [Member] \| Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Leverage position	200.00%